LYS-5 2012-1 Soru=30

What could be the best title for the passage?

To see the big obstacle facing renewable energy, look
at Denmark. It has some of the world’s largest wind
farms. Yet because consumer demand for electricity is
often lowest when the winds blow hardest, Denmark
sometimes has to sell its extra electricity to
neighbouring countries at low prices ‒ only to buy
energy back when demand rises, at much higher prices.
Companies in Texas face a similar mismatch between
supply and demand; they sometimes have to pay
customers to take energy from their windmills and solar
farms. In theory, the wind and sun could supply the US
with all the electricity it requires. In practice, however,
both sources are too unreliable to supply more than
20% of a region’s total energy capacity. Beyond that
point, balancing supply and demand becomes too very
difficult. What is needed are cheaper and more efficient
ways of keeping power in reserve so that it can be
accessed when the winds are not blowing and the sun
is not shining.